Meltdown 101: A brief glossary of nuclear terms
Fissionable material is fed into the reactor core in the form of thimble-sized pellets. Pellets are loaded into 15-foot metal rods which are loaded into the reactor in bundles of 82 called fuel assemblies. Periodically, fuel rods are removed from the reactor core and replaced with new fuel.
According to the United States Nuclear Regulatory Commission, modern reactor cores can contain up to 10 million pellets.